this was something i failed to clarify better on the English version, that, by vocabulary, made more sense on the original text:
what you said is absolutely right, the tanker's roles should be interpreted more as _seats_ on the sherman than actual strict classes. if a guy dies, any other tanker will have to shift from their seat to the felled crewman's position and do their job for them — funilly enough, that's kinda how tanker hierarchy worked back then, you started as a dipshit bow gunner and worked your way through the stations, learning how each one worked, settling on the one your seniors picked for you.
you can see this exact rule interaction implied on the downtime activities, there it says that "if you're driving you can't sleep, or be patched up or so", not that the driver should go fuck themselves, y'know?
tl;dr - tankers are not locked to their roles, only fulfill them better then anyone else. if someone dies, any other tanker shall do their job, but without the focuses or sheet upgrades that tanker had.